Mechanomyographic and electromyographic responses during fatigue in humans: influence of muscle length

Mechanomyography (MMG) provides a measure of muscle mechanical changes duri ng contractions. The purpose of this study was to quantify alterations in M MG signals during fatigue at two muscle lengths. Comparisons with electromy ographic (EMG) recordings were made. A group of 13 subjects performed isome tric dorsiflexions (50% of maximum for 60 s) at 40 degrees of plantarflexio n (long, l(l)) and 5 degrees of dorsiflexion (short, l(s)). The mean power frequency of the EMG ((-)f(EMG)) and MMG ((-)f(MMG)) signals and the mean r ectified MMG (r (MMG) over bar) and EMG (r (EMG) over bar) were determined over each 1-s period, normalized to the respective maximal value, regressed against time, and the resulting slopes (units = %max . s(-1))were analyzed . The r (MMG) over bar slopes were larger (P = 0.007) at l(l) compared to l (s) [mean l(l) 0.50 (SD 0.26). mean l(s) 0.27 (SD 0.16)], however there wer e no differences (P = 0.24) between mean (-)f(MMG) slopes [l(l) -0.10 (SD 0 .16), l(s) -0.16 (SD 0.11)]. Similarly, r (EMG) over bar slopes were larger (P = 0.001) at l(l) versus l(s) [l(l) 0.26 (SD 0.13), l(s) 0.08 (SD 0.15)] and there were no differences (P = 0.89) between mean (-)f(EMG) slopes [l( l) -0.15 (SD 0.14), l(s) -0.14(SD 0.12)]. At 5 s following the exercise to fatigue mean MVC (units = %max) were not significantly different between l( l) and I, [P = 0.08; l(l) 78.8 (SD 9.1), l(s) 85.2 (SD 6.0)]. These results showed that during fatiguing contractions, MMG and EMG amplitudes increase d while frequency characteristics decreased at both muscle lengths. The cha nge in r (MMG) over bar and r (EMG) over bar was greater at l(l) but no dif ferences in (-)f(MMG) or (-)f(EMG) slopes occurred between lengths. These r esults would suggest that larger increases in motor unit recruitment occur with time during fatigue at l(l) compared to l(s).